Fibers with excellent elasticity are needed to manufacture a variety of fabrics which are used, in turn, to manufacture a variety of durable articles such as, for example, sport apparel, furniture upholstery and hygiene articles. Elasticity is a performance attribute, and it is one measure of the ability of a fabric to conform to the body of a wearer or to the frame of an item. Preferably, the fabric will maintain its conforming fit during repeated use, e.g., during repeated extensions and retractions at body and other elevated temperatures (such as those experienced during the washing and drying of the fabric).
Fibers are typically characterized as elastic if they have a high percent elastic recovery (that is, a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three important properties: (i) a low percent permanent set, (ii) a low stress or load at strain, and (iii) a low percent stress or load relaxation. In other words, elastic materials are characterized as having the following properties (i) a low stress or load requirement (i.e., a low biasing force) to stretch the material, (ii) no or low relaxing of the stress or unloading once the material is stretched, and (iii) complete or high recovery to original dimensions after the stretching, biasing or straining force is discontinued.
Heat-setting is the process of exposing a fiber or article made from the fiber, e.g., a fabric, while under dimensional constraint to an elevated temperature, typically a temperature higher than any temperature that the fiber or article is likely to experience in subsequent processing (e.g., dyeing) or use (e.g., washing, drying and/or ironing). The purpose of heat-setting a fiber or article is to impart to it dimensional stability, e.g., prevention of or inhibition against stretching or shrinkage. The structural mechanics of heat-setting depend upon a number of factors including fiber morphology, fiber cohesive interactions and thermal transitions.
Elastic fibers, both covered and uncovered, are typically stretched during knitting, weaving and the like, i.e., they experience a biasing force that results in an elongation or lengthening of the fiber. Large degrees of stretch, even at ambient temperature, produces a permanent set, i.e., part of the applied stretch is not recovered when the biasing force is released. Exposure of the stretched fiber to heat can increase the permanent set, thus resulting in a fiber that is “heat-set”. The fiber thus assumes a new relaxed length which is longer than its original, pre-stretched length. Based on the conservation of volume, the new denier, i.e., fiber diameter, is lowered by a factor of the permanent stretch, i.e., the new denier is equal to the original denier divided by the permanent stretch ratio. This is known as “redeniering”, and it is considered an important performance attribute of elastic fibers and fabrics made from the fibers. The processes of heat-setting and redeniering a fiber or an article is more fully described in the heat-setting experiments reported in the Preferred Embodiments.
Spandex is a segmented polyurethane elastic material known to exhibit nearly ideal elastic properties. However, spandex exhibits poor environmental resistance to ozone, chlorine and high temperatures, especially in the presence of moisture. Such properties, particularly the lack of resistance to chlorine, causes spandex to pose distinct disadvantages in apparel applications, such as swimwear and in white garments that are desirably laundered in the presence of chlorine bleach.
Moreover, because of its hard domain/soft domain segmented structure, a spandex fiber does not reversibly heat-set. In spandex, heat setting involves molecular bond breaking and reformation. The fiber does not retain any “memory” of its original length and, consequently, it does not have any driving force to return it to a pre-heat orientation. The heat setting is not reversible.
Elastic fibers and other materials comprising polyolefins, including homogeneously branched linear or substantially linear ethylene/∀-olefin interpolymers, are known, e.g., U.S. Pat. Nos. 5,272,236, 5,278,272, 5,322,728, 5,380,810, 5,472,775, 5,645,542, 6,140,442 and 6,225,243. These materials are also known to exhibit good resistance to ozone, chlorine and high temperature, especially in the presence of moisture. However, polyolefin polymer materials are also known to shrink upon exposure to elevated temperatures, i.e., temperatures in excess of ambient or room temperature.
The concept of crosslinking polyethylene to increase its high temperature stability is known. WO 99/63021 and U.S. Pat. No. 6,500,540 describe elastic articles comprising substantially cured, irradiated or crosslinked (or curable, irradiatable or crosslinkable) homogeneously branched ethylene interpolymers characterized by a density of less than 0.90 g/cc and optionally containing at least one nitrogen-stabilizer. These articles are useful in applications in which good elasticity must be maintained at elevated processing temperatures and after laundering.